A pulsed linear fiber laser with passive mode locking have been disclosed in the prior art (see, for example, WO 2004/059806), where the mode-locking function is performed by a saturable absorber made of carbon nano-tubes layered on a totally reflective mirror of the laser cavity. The primary drawback of such saturable absorbers, including those based on carbon nano-tubes, is their degradation under a high intensity of laser radiation which is required to saturate their absorption. Therefore, a lifetime of the saturable absorbers typically does not exceed the period of several hundred-several thousand hours. Moreover, the preparation of a uniform matrix of carbon nano-tubes and applying it to laser mirrors relies on a complex technological processes, which is not always possible even in the laboratory conditions.
A pulsed linear fiber laser with passive mode locking have been disclosed in the prior art (see, for example, US20090003391 where a low-repetition-rate ring-cavity passively mode-locked fiber laser) is disclosed. The mode-locking function is performed by a saturable absorber made of carbon nano-tubes or a semiconductor material.
The linear (non-circular) laser configuration employs reflectors that limit the laser radiation spectrum also limiting the shortest possible duration of output pulses. Moreover, the linear laser cavities also give rise to the effect of spatial hole burning within the gain medium, consequently leading to the presence of weakly competing components in the laser output spectrum. Interaction between these components generates au increased level of output intensity noise.
A linear fiber laser that is passively mode-locked by a semiconductor saturable absorber, such as one disclosed in U.S. Pat. No. 6,097,741, is the closest prior art to the present invention. The major drawback of this solution is degradation of semiconductor absorbers under a high incident intensity of laser radiation. Lifetime of semiconductor saturable absorbers cannot exceed several thousand hours. Moreover, the fabrication of a semiconductor saturable absorber and its integration into a mirror poses complex technological problems that demand expensive equipment and materials along with a high qualification of personnel.
A disc laser that can be mode-locked by a Kerr lens is also known in the prior art. The Kerr lens is an optical element which is placed at the waist of the intra-cavity laser beam (see, for example, WO2013050054). Unfortunately, this solution is only applicable to lasers with a disc-shaped active medium and cannot be used for fiber lasers' mode locking.
A pulsed ring fiber laser that is mode-locked due to the effect of non-linear polarisation evolution of radiation within a non-polarization-maintaining laser cavity fiber, such as disclosed in US 20100220751, is s the closest prior art to the present invention. However, such solution suffers from several disadvantages.
Firstly, the reported mode-locked operation is triggered, by an alignment of three phase plates (two quarter-wave and one half-wave plate), which poses algorithmic problems in their simultaneous adjustment. Usually one keeps adjusting them randomly until a mode locking spontaneously triggered combination is reached.
Secondly, such a passively mode-locked (due to the effect of non-linear polarization evolution) fiber laser can accommodate various types of mode-locked operation. In order to identify the triggered regime and determine the parameters of generated pulses, it is necessary to use a very specific measuring equipment every time after the phase plates are adjusted for mode locking triggering.
Third, such a passively mode-locked fiber lasers are relatively sensitive to the ambient temperature. A change as small as several degrees requires a new phase plate adjustment procedure to restart the mode locking.
There is a need for improved technique for mode-locked operation of fiber lasers.